In the field of non-impact printing, the most common types of printers have been the thermal printer and the ink jet printer. When the performance of a non-impact printer is compared with that of an impact printer, one of the problems in the non-impact machine has been the control of the printing operation. As is well-known, the impact operation depends upon the movement of impact members, such as print hammers or wires or the like, which are typically moved by means of an electromechanical system and which may, in certain applications, enable a more precise control of the impact members.
The advent of non-impact printing, as in the case of thermal printing, brought out the fact that the heating cycle must be controlled in a manner to obtain maximum repeated operations. Likewise, the control of ink jet printing, in at least one form thereof, must deal with rapid starting and stopping movement of the ink fluid from a supply of the fluid. In each case of non-impact printing, the precise control of the thermal elements and of the ink droplets is necessary to provide for both correct and high-speed printing.
In the matter of ink jet printing, it is extremely important that the control of the ink droplets be precise and accurate from the time of formation of the droplets to depositing of such droplets on paper or like record media and to make certain that a clean printed character results from the ink droplets. While the method of printing with ink droplets may be performed either in a continuous manner or in a demand pulse manner, the latter type method and operation is disclosed and is preferred in the present invention in applying the features of the present invention. The drive means for the ink droplets is generally in the form of a crystal or piezoelectric type element to provide the high speed operation for ejecting the ink through the nozzle while allowing time between droplets for proper operation. The ink nozzle construction and operation must be of a nature to permit fast and clean ejection of ink droplets from the print head.
Additionally, in an ink jet printer, it is considered a basic requirement to provide some type of means for reducing or substantially eliminating any air or other type gas bubbles that may form in the ink fluid. In a drop-on-demand ink jet printing device, the presence of such gas bubbles has been suspected to be a primary factor affecting performance and, in certain instances, even inhibiting ejection of ink droplets. It has been determined that the presence of air in or passing through the nozzle of the piezoelectric element affects ink droplet ejection and also that nozzle surface conditions, wherein various forms of contamination such as lubricants, detergents, wetting agents, smoke, paper dust or other materials, have an effect on and may even provoke the ingestion of air through the nozzle. Further, it has been observed that air can be ingested into the ink channel either by disturbing the supply conduit leading from the ink reservoir to the ink jet print head, as represented by the piezoelectric element, or air can be ingested into the system by subjecting the print head to axial acceleration.
It is also known that the size of the air bubble or the amount of air within the system affects the ejection of ink droplets in the manner wherein a small air bubble or amount of air may increase or decrease the velocity of the ink droplets being ejected, whereas a large air bubble or amount of air may block the ink channel and thereby inhibit ejection of ink droplets. It is further noted that purging the system normally removes the larger amounts of air that tend to block the ink channel, however, smaller bubbles or amounts of air may remain in the channel and thus affect the velocity of the ink droplets. The reduction or substantial elimination of gaseous bubbles in the ink fluid is a desirable feature in the operation of an ink jet printing system.
Representative documentation in the field of ink jet printing and in preventing gaseous bubbles by heating the ink fluid used during the printing operation includes U.S. Pat. No.3,179,042, issued to M. Naiman on Apr. 20, 1965, which discloses an ink steam generating device consisting of a pair of electrodes which are immersed in the ink to produce a high I.sup.2 R loss. Current passes through the ink in a gap between the electrodes causing generation of heat which will vaporize that portion of the ink contained between the electrode tips, and which vapor will tend to expand and exert a sufficient pressure on the ink directly above the tips to force individual droplets of ink from the tube to the paper. The ends of the electrodes are placed within a tube in which ink is supplied through a duct from an ink supply. Ink is maintained near its boiling point by means of a heater placed within the ink supply, and the ink within the area of the electrode gap is caused to become vaporized or ionized and to cause any trapped gases to expand. The expansion of gases causes a great force on the ink above the electrodes and ink is propelled out the open aperture of the tube in the form of droplets.
U.S. Pat. No. 4,007,684, issued to R. Takano et al. on Feb. 15, 1977, discloses an ink liquid warmer provided in the ink supply system to maintain the ink at a predetermined temperature at the nozzle without regard to temperature condition of the ink supply or to ambient conditions. The predetermined ink liquid temperature maintains the viscosity and surface tension of the ink liquid at a constant value. The system includes thermistors which are stable temperature devices and which maintain the predetermined temperature.